The present invention relates to a method of correcting the gradation characteristic of a printer and a color patch image used for this correction.
In the case that an image is produced by a computer or an image is obtained by an optical read-out, in order to print out a printed matter corresponding to such an image with intended colors, correction of image signals is conducted in such a way that the gradation characteristic (the characteristic of output gradation values to input gradation values (image signal values)) of a printer is measured, gradation correction characteristics (for example, it is saved in the form of printer profile mentioned later and utilized) is produced based on the above measurement result, and image signals are corrected by the produced gradation correction characteristics.
Usually, measurement of the gradation characteristic of a printer is conducted by a process of printing a color patch of predetermined colors with a printer and a process of measuring the printed color patch by a color measuring device. It is desirable that the color patch is produced for all of colors capable of being expressed with image signals provided to the printer. However, there is a large number of colors capable of being expressed (combination of gradation values capable of being made by each color component of image signals). For example, in the case of a full color printer employing four color components of Y (Yellow), M (Magenta), C (cyan) and K (black), if it is assumed that each color ahs 256 gradations, there are combinations of gradation values more than 4 billion (4th power values of 256). For this reason, a color patches are produced for only a small portion of these combinations, and the gradation characteristics of intermediate color among the produced color patches are obtained by various kinds of interpolation operations.
FIG. 23 shows an example of the gradation characteristic of a printer obtained for one color component by the interpolation. Among the gradation values (256 kinds of gradation values in the case of 8 bits) capable of being taken by color components, several points are chosen as gradation representative values a1 to a6, and color patches corresponding to gradation representative values a1 to a6 are printed out and measured, whereby measurement values indicated with a circle mark in the drawing can be obtained. Subsequently, it is assumed that the characteristic changes continuously (or linearly), intermediate portions among the measurement values are obtained by interpolation, whereby the gradation characteristic A shown in the drawing can be obtained. Although various gradation representative values can be taken, the number of gradation representative values is set in the range of 5 to 9 for almost one color component. Actually, color patches are for all of the combinations of gradation representative values among color components, or for combinations selected further from these combinations.
FIG. 24 shows an example of a gradation correction characteristic B produced based on the gradation characteristic of a printer. The gradation correction characteristic B is a conversion characteristic of input gradation values (input image signal values) required to obtain predetermined output gradation values with the printer. At the time of printing a color chart image, image signals are obtained from input image signals by gradation conversion based on the gradation correction characteristic and the obtained image signals are outputted to a printer. Here, it is also desirable that an original gradation correction characteristic is defined for all of the number of gradations capable of being taken by image signal values, for example, for all of 256 gradations in the case of 8-bit images. However, if the gradation correction characteristics defied for all of the number of gradations are held as data, since the data becomes a huge amount of information, this is not practical. Then, gradation representative values are set up discretely, a LUT (Look Up Table) in which each gradation representative value is correlated with a gradation value after conversion is produced and saved, an intermediate vale between neighboring gradation representative values is interpolated under the assumption that the characteristic changes linearly (or continuously). Although the number of gradation representative values can be taken variously, the number of gradation representative values is set in the range of 15 to 33 for almost one color component, and combinations with regard to all of these color components are constituted.
Further, dispersion may be caused in color reproducibility depending on printing positions within a chart even the same patch. In order to cope with this problem, there is a method (refer Patent Document 1) in which color chart is made to include measurement-use patches for the measurement of printing characteristics and plural correction-use same color patches arranged at predetermined positions, the data of the color chart are printed, the printed color chart is measured, the degree of dispersion in the color measurement values of the measurement-use patches is calculated, the color measurement values of the measurement-use patches is corrected, and the profile of a printer is produced based on the corrected color measurement values of the measurement-use patches and the color values of the color chart data (refer to Patent Document 1: Japanese Patent Unexamined Publication No. 2001-320592).
Further, there is a profile producing method in which the number of outputted sheets of charts is reduced as compared with the case where many color patches are arranged, by measuring a gradation pattern for each of minute regions. For example, there is a method (refer Patent Document 2) in which gradation is formed such that lightness changes continuously, the gradation is subjected to color measurement for each minute region to obtain color measurement result data and a correspondence relationship between the color information of a reference pattern image and the obtained color measurement result data (refer to Patent Document 2: Japanese Patent Unexamined Publication No. 2005-318460).
Furthermore, there is an apparatus (refer Patent Document 3) which produces a chart in which optimal color patches corresponding to the classification of printers, such as an electro-photographic type and an ink-jet type and conducts color correction based on the chart (refer to Patent Documents 3: Japanese Patent Unexamined Publication No. 2008-72343).
A plurality of color patches corresponding to plural gradation representative values are printed with a printer, and then, the gradation characteristic of the printer is obtained by the interpolation of those measurement values. The resultant gradation characteristic of the printer is made based on the presupposition that the gradation characteristic changes continuously (or linearly) in intermediate portions among the gradation representative values. However, since most digital printers have realized a continuous gradation by pseudo multi gradation processing, there is a possibility that the discontinuity of gradation so called a tone jump occurs in limited gradation value depending on a technique related to an image forming condition.
FIG. 25 shows an example of the gradation characteristic of a printer in which a tone jump has occurred. An axis of abscissa represents the image signal values (input gradation values) given to the printer, and an axis of ordinate represents the gradation values (output gradation values) obtained by the actual measurement of a printed image. A tone jump J has occurred in a portion at which the gradation characteristics C indicated with a solid line becomes discontinuous. In the drawing, for comparison, a dotted line represents a gradation characteristic C′ in which there is no tone jump.
For example, in the case that the pseudo multi gradation processing is being performed by a step of changing of the size of dot to be printed out, when the size of dot becomes large (or small) gradually, a tone jump is caused by the large change of gradation at the moment that neighboring dots come in contact with each other (or separate from each other). Successively, the influence of the tone jump appears as discontinuous in optical density (actual measurements, of course, for example, brightness, or reflectance ratio) of a printed image corresponding to image signal values.
With regard to the occurrence position of a tone jump, a tone jump may occur at various positions depending on a control mechanism of a printer. Further, even in the same printer, the occurrence positions (image signal values) of a tone jump changes in accordance with a process of correcting characteristic fluctuation of a printer.
If the gradation value of a color patch resides in the vicinity of the image signal value on which a tone jump occurs (in the example of FIG. 25, a tone jump occurs on the gradation representative value a3), a large change arises in the output gradation value of a color patch according to the condition of the printer at the time of the output of a patch, and the large change causes large influence onto the gradation characteristics of the printer obtained by the interpolation of the output gradation value.
FIG. 26 shows the example.
In the case that a tone jump occurs in the vicinity of the gradation representative value “a”, if the characteristic of a printer changes slightly, a large change appears in the color measurement values measured across the tone jump. FIG. 26 shows the gradation characteristic F2 of a printer in the case that the gradation value at the time of the occurrence of a tone jump J is a gradation value La slightly lower than the gradation representative value “a” and the gradation characteristic F1 of the printer in the case that the gradation value at the time of the occurrence of a tone jump J is the gradation value Ua slightly higher than the gradation representative value “a”. In this way, if the occurrence position of a tone jump J is changed across the gradation representative value “a”, a large difference is caused in the gradation characteristics F1, F2 obtained by the interpolation of the measurement values related to gradation representative value “a”. Further, the difference becomes very large as shown with a region E which is covered with gray and enclosed with lines connecting points of the gradation representative value “a” and another two gradation representative values (indicated with respective dashed lines at the right and left positions in the drawing) located before and after the value “a” on the gradation characteristics F1 and F2. This phenomenon is caused not only by a tone jump, but also by the case that there exists a gradation region in which a gradation characteristics is not discontinuous, but changes suddenly.
Patent Documents 1 mentioned above discloses a method of corresponding to variation in color reproducibility depending on printing positions, and Patent Documents 3 discloses a method of determining the arrangement patterns of color patches in accordance with the type and characteristic of an output device. However, both of these documents do not disclose a technique to reduce the influence of a tone jump and the like which appear in the gradation characteristic of a printer obtained by the interpolation of the measurement results of the color patches.
Further, as disclosed in Patent Documents 2, if a gradation pattern is measured in a number of measurement positions for each minute region, even if a tone jump and the like occur, errors by interpolation may be reduced. However, a measuring device with a high degree of accuracy and high resolution is needed for the measurement for such minute regions. Further, if there exist minute dirt, dust, blemish, and the like on printed gradation patterns, the measurement for such minute regions may receive the influence by them easily. Furthermore, in the case that a specific pattern appear in a gradation pattern by the pseudo multi gradation processing for a printer, the specific pattern causes large influence on the measurement result of minute region, and the exact measurement becomes difficult.